One of the main problems which rolling mill operators complain about is how to control the axial position of the slab with respect to the axis of the first rolling stands located downstream of the heating furnace.
It is well-known that the slab as it emerges from the continuous casting machine, since it must be subjected to the processes of extraction, pre-rolling and straightening, rarely remains correctly aligned with the axis of feed; this causes considerable problems when the slab enters the rolling stands and during the rolling steps.
Moreover, as it travels inside the tunnel furnaces, either the heating furnace or the temperature maintenance furnace, the slab may be subject to lateral displacements which send it out of line.
If the slab arrives misaligned with respect to the axis of the first stand, rolling becomes difficult, particularly when thin diameters are being rolled.
In fact, in order to compensate for this misalignment after the slab has entered the stand, and to ensure that it enters the downstream stand correctly, it is necessary to act on the horizontal positioning of the first stand, which may have negative effects on the symmetry of the profile in the cross section of the slab itself.
While this does not create particular problems when the partly rolled product has a greater thickness, for example above 2 mm, in the case of thinner products there are considerable problems of quality, inasmuch as it becomes extremely difficult, if not impossible, to recover the difference in thickness between one side and the other, when the product is 0.6-0.8 mm thick.
In the case of thin products the transverse sliding of the material is very difficult to obtain and in any case it causes errors in planarity deriving from a differentiated stretching of the material.
In order to solve these problems, at least partly, solutions known to the art employ the action, either individually or in combination, of lateral guides, of the descaling assembly or the rolls or assemblies to finish the edges, which are arranged between the outlet of the heating furnace and the inlet to the stand, in order to obtain the progressive axial alignment of the slab and the rolling axis.
These solutions have shown themselves to be only partly efficacious, for a variety of reasons.
First of all, there is a technological requirement whereby the entrance to the stand cannot be too distant from the outlet of the furnace (the typical maximum value is around 14-20 meters), to prevent the excessive cooling of the slab to below the optimum rolling temperature.
For this reason it is necessary to obtain a great displacement of the slab per unit of length of the plant, in order to obtain the desired alignment in correspondence with the inlet to the stand.
Guide systems as are known to the art, however, are not able to obtain such values of displacement, and therefore they do not enable the desired alignment to be reached in the little space available between the furnace and the stands, which is imposed by technological constraints.
Lateral guides as are known to the art, moreover, occupy about 10 meters in length of the segment between the furnace and the stand, and define a transit width which is greater than the width of the slab, on both sides, by at least 25 mm per side, up to as much as 50 mm per side. Therefore, the alignment of the slab is imprecise by values of .+-.25-50 mm.
Moreover, the rollers which refine the edges, or edgers, cannot act upon the edges of the slab for more than about 10 mm per side.
All these factors make it impossible to centre the slab if the slab arrives misaligned with respect to the rolling axis beyond a minimum value which can be compensated, and which can be estimated in the region of .+-.10 mm.
There is also the further problem concerning the transport rollers inside the heating furnace.
In order to withstand the extremely high temperatures of up to 1110-1200.degree. C. inside the furnace, these rollers are structured with cooled rolls which support disks made of refractory material which is mechanically very delicate, so that even a slight transverse displacement of the advancing slab causes considerable damage and puts the disks out of action very quickly.
It is therefore highly inadvisable to make the slab translate laterally when it is inside the furnace.
JP-A-62235429 teaches to provide nozzles arranged above and below the rolled stock passing through, which deliver a jet of gas in the opposite direction to the direction of feed of the rolled stock.
The nozzles are arranged in a zig-zag conformation and exert an action of mechanical displacement on the rolled stock if it is not centered with respect to the relative feeding means.
This device makes possible to obtain only limited adjustments in the position of the rolled stock, and moreover it may cause unacceptable modifications in the surface temperature conditions thereof.
EP-A-416356 describes an alignment station for rolled products arranged between the drawing-straightening assembly which acts on the rolled stock emerging from the continuous caster and the shears which shear the rolled stock into segments which are then sent to the temperature equalisation furnace.
The alignment station consists of a supporting roller, positioned under the plane on which the rolled stock is fed, with bearings which are connected to a relative vertical piston suitable to incline the roller to one side or the other so as to correct any possible lateral displacement of the rolled stock.
Alternatively, the alignment station comprises at least a burner, or at least a sprayer nozzle, cooperating with at least one edge of the rolled stock in order to align the rolled stock, either by exploiting the dilation caused by heating, or by exploiting the shrinkage caused by cooling.
The fact that the alignment station is positioned upstream of the shears assembly and of the furnace creates the problem that, precisely during the shearing cycle or during the heat treatment in the furnace, the segment of rolled stock becomes misaligned and arrives in correspondence with the first stands of the rolling train in an out-of-center condition.
Moreover, the inclusion of a single alignment roller can make it impossible to correct misalignments of the rolled stock of a certain entity, inasmuch as the lowering or raising of one side of the roller with respect to the other is limited by the overall height of the plane of feed.
Furthermore, EP'356 does not mention any systems to control the position of the slab with respect to the axis of the rolling stands, nor any feedback systems which govern the alignment means and condition the functioning thereof in the event of misalignments being found downstream.
The present applicant has designed and tested this invention to overcome these shortcomings which cause serious operating and technological problems, and problems of quality, in the rolling of flat products, particularly thin flat products of less than 2 mm and down to 0.8-0.5 mm.